The present invention generally relates to an optical projection apparatus for producing semiconductor devices or the like, and particularly relates to an optical projection apparatus with the function of controlling laser coherency by use of an excimer laser.
Conventionally, the optical projection apparatus using a laser has been discussed, for example, in "Development in Semiconductor Micro-lithography", SPIE (The Society of Photo-Optical Instrumentation Engineers), Vol. 174, pp. 28-36, 1979. In this document, light emitted from a krypton laser 1 is scanned by scanning mirrors 3 and 6 in the x- and y-direction as shown in FIG. 2a, and the scanned light is converged to the position of an entrance pupil 10 of a projection lens 11 by means of a condenser lens 8 so as to be scanned in the x- and y-direction as shown in FIG. 2b. A mask 9 is disposed behind the condenser lens 8 so that a pattern of the mask 9 is focused on an image plane 12 by the projection lens 11. The reference numerals 2, 4, 5, and 7 designate elements constituting a focusing optical system. The ratio of the scanning area a of a scanning spot shown in FIG. 2b to the size R of the entrance pupil 10, that is, .sigma.=a/R, is called a coherence factor. With respect to the coherence factor .sigma., .sigma.=0 and .sigma.=.infin. correspond to incoherent radiation and coherent radiation respectively. In the coherent radiation, the image contrast can be made high, while the limiting resolution cannot be made high. In the incoherent radiation, on the contrary, the limiting resolution can be made high, while the image contrast becomes low. Therefore, generally, the coherence factor .sigma. is selected to be about 0.7. In the system shown in FIGS. 2a and 2b, the entrance pupil 10 is scanned by a scanning spot so that an image of the mask pattern 9 is formed on the image plane 12 with respect to every scanning position. Thus, the image of the mask pattern 9 is recorded as superposition of the light intensity, not of the light amplitude, on a photosensitive material mounted on the image plane 12. As the result, interference noises (speckle noises appearing on the image plane 12) due to laser coherency are remarkably reduced.